Wells are generally drilled into the ground to recover natural deposits of oil and gas, as well as other desirable materials, that are trapped in geological formations in the Earth's crust. A well is typically drilled using a drill bit attached to the lower end of a “drill string.” Drilling fluid, or “mud,” is typically pumped down through the drill string to the drill bit. The drilling fluid lubricates and cools the drill bit, and it carries drill cuttings back to the surface in the annulus between the drill string and the borehole wall.
It is often desirable to have information about the subsurface formations that are penetrated by a well. For example, one aspect of standard formation evaluation relates to the measurements of the formation pressure and formation permeability. These measurements are essential to predicting the production capacity and production lifetime of a subsurface formation.
One technique for measuring formation properties includes lowering a “wireline” tool into the well to measure formation properties. A wireline tool is a measurement tool that is suspended from a wire as it is lowered into a well so that is can measure formation properties at desired depths. A typical wireline tool may include a probe that may be pressed against the borehole wall to establish fluid communication with the formation. This type of wireline tool is often called a “formation tester.” Using a probe, a formation tester can measure the pressure of the formation fluids, generate a pressure pulse to determine the formation permeability, and withdraw a sample of formation fluid for later analysis.
In order to use a wireline tool, the drill string must be removed from the well so that the tool can be lowered into the well. This is called a “trip” downhole. Because of the great expense and rig time required to “trip” the drill pipe, wireline tools are generally used only when the information is absolutely needed or when the drill string is tripped for another reason, such as changing the drill bit. Examples of wireline formation testers are described, for example, in U.S. Pat. Nos. 3,934,468; 4,860,581; 4,893,505; 4,936,139; and 5,622,223.
Another technique for measuring formation properties uses measurement tools and devices that are positioned near the drill bit in a drilling system. Measurements are made during the drilling process. A variety of downhole drilling tools, such as logging-while-drilling tools and measurement-while-drilling tools, commercially are available. “Logging-while-drilling”(“LWD”) is used to describe measuring formation properties during the drilling process. Real-time data, such as the formation pressure, allows the driller to make decisions about drilling mud weight and composition, as well as decisions about drilling rate and weight-on-bit, during the drilling process. It is noted that LWD and “measurement-while-drilling”(“MWD”) have different meanings to those having ordinary skill in the art. MWD typically refers to measuring the drill bit trajectory as well as borehole temperature and pressure, while LWD refers to measuring formation parameters, such as resistivity, porosity, permeability, and sonic velocity, among others. The distinction between LWD and MWD is not germane to the present invention, thus, this disclosure does not distinguish between the two terms.
Formation evaluation while drilling tools capable of performing various downhole formation testing typically include a small probe or pair of packers that can be extended from a drill collar to establish fluid communication between the formation and pressure sensors in the tool so that the formation fluid pressure may be measured. Some existing tools use a pump to actively draw a fluid sample out of the formation so that it may be stored in a sample chamber in the tool for later analysis. Such a pump is typically powered by a battery or by a generator in the drill string that is driven by the mud flow.
What is still needed, therefore, are techniques for downhole formation evaluation while drilling tool that are more reliable and efficient, yet able to conserve space in a downhole drill collar.